What was actually said in the original Japanese was that it was the last 1-series DSLR. And I think it only became the Mk III because the Mk II used an orphaned memory card standard.
Nothing was said about the more mainstream 5-series either way. Reading the tea leaves, I would not be surprised if the Mk IV was the last, but nothing was said.
The modern 1-series is far more appropriate to be replaced by mirrorless because of the kind of action photography it is mostly used for.
My opinion:
M series is here to stay for at least 2-3 years. Possibly a new body will come, but it'll be an incremental upgrade rather than anything radical.
Budget APS-C (Rebel) DSLRs will also stay for at least 3 years, as they are much cheaper to produce than RF models, and will feed economy buyers into the Canon system. Most likely there will be a new model, but again, nothing radical.
Mid-range DSLRs such as 90D and 5DMkiv are probably still in production as there is still enough demand, but they'll probably be phased out in the next couple of years.
1Dxiii will continue to be made until demand for it disappears. Canon's claim that the 1Dxiii is still the current flagship is designed purely to calm recent buyers who might be upset that the R3 is better and cheaper. The R3 is hugely superior in almost every regard, it's the top model in the Canon range, and therefore it's the current flagship. It will considerably outsell the Nikon Z9 and the Sony A1, despite them being arguably better specified.
The "R1" is at least a year away.
Next RF camera will be the "R7" which will probably be a mirrorless 90D, with animal-eye AF and 20fps.